1. Field of the Invention
An electromagnetic brake assembly for controlling rotary inertia forces in a road vehicle powertrain.
2. Background Art
A typical powertrain for a road vehicle, particularly a heavy-duty truck, includes an internal combustion engine and a multiple-ratio power transmission mechanism, together with a master disconnect clutch for establishing and disestablishing power flow paths from the engine through the multiple-ratio transmission mechanism. Ratio changes in the transmission mechanism are accomplished by shifting clutch elements into and out of engagement with companion gear elements. The ratio changes occur during a shift interval in which the vehicle operator disengages the master clutch.
A ratio change shift sequence typically involves disengagement of the master clutch to interrupt power flow from the vehicle engine to the torque input shaft of the transmission as the transmission clutch elements are selectively engaged and disengaged. When the master clutch is disengaged, a torque input shaft for the transmission must decelerate so that the gear elements of the on-coming torque flow path are generally in synchronism.
A brake may be used to facilitate shifting of the transmission gearing by decelerating the transmission torque input shaft, thereby decreasing the time required to accomplish a ratio shift and improving shift quality. The torque input shaft brake is especially useful when the vehicle driver initiates a shift from neutral to low ratio or from neutral to reverse following master clutch disengagement.
It is known in the art to provide a transmission input shaft brake that includes a friction member connected drivably, such as by splines, to the transmission torque input shaft. The transmission master clutch is disengaged by a master clutch release mechanism so that when the master clutch is disengaged, the release mechanism will apply a brake engaging force on the transmission input shaft brake. Friction brake elements of the input shaft brake thus are activated into frictional engagement, thereby creating a frictional drag torque that decelerates the transmission input shaft.
U.S. Pat. No. 7,000,748 discloses a transmission input shaft brake with an electromagnetic brake actuator. That patent is assigned to the assignee of the present invention. The electromagnetic brake disclosed in the '748 patent comprises an armature that is secured to the transmission input shaft adjacent a friction surface formed on an adjacent transmission housing wall. When the brake is energized, the armature frictionally engages a stationary friction surface on the transmission housing wall, thereby retarding or preventing rotation of the transmission torque input shaft at the outset of a ratio shift.
The electromagnetic brake of the '748 patent creates a magnetic flux flow path that is defined in part by a brake armature. The flux flow path envelopes portions of the transmission, including the transmission input shaft, a transmission input shaft bearing and bearing cover, and portions of a driver operated master clutch release mechanism.
The electromagnetic input shaft brake disclosed in the '748 patent includes a housing, which may replace a transmission input shaft bearing cap typically found on heavy-duty transmissions. The electromagnetic brake includes coil windings that are placed close to the input shaft to reduce the length of the coil windings and to reduce the amount of copper required in the manufacture of the coil. Typically, the electromagnetic brake is strategically positioned to minimize the space required to accommodate it in the transmission assembly.
The magnetic lines of flux created as the transmission input shaft brake is activated pass through the transmission input shaft and surrounding portions of the transmission that are of high carbon content, which may be magnetized following a period in which the transmission input shaft brake is frequently activated. It is possible, for example, for the transmission input shaft to be partially magnetized with a permanent residual magnetic intensity of about 0.5 to 1.0 Tesla. The transmission housing, which typically is formed of cast aluminum or cast iron with a low carbon content, does not readily become magnetized because those materials are relatively poor conductors for magnetic flux fields. The input shaft itself, however, as well as the bearing elements and other transmission elements and seal covers, are formed of high carbon steel and are in close proximity to the input shaft brake.
The return flux flow path in an arrangement of this type typically includes an armature plate of the input shaft brake, which may be a solid disk design because of its ease of manufacture and its low cost.
Because of partial or residual magnetization of transmission components in proximity to the input shaft brake, ferrous particles in an operating environment for the transmission can be attracted to rotary portions of the transmission and damage transmission bearings, seals and other transmission components.
The electromagnetic brake disclosed in the co-pending 069 patent application, now U.S. Pat. No. 7,318,515. which is assigned to the assignee of the present invention, includes a clutch hub that is splined or otherwise secured to a torque input shaft of a multiple-ratio transmission. The hub is drivably connected to a torque output side of the master clutch. The electromagnetic brake includes an armature that is drivably connected to a brake (clutch brake) hub through a flexible drive plate. Electromagnetic coil windings are positioned at a relatively increased radial distance from the axis of the torque input shaft to reduce the problem of residual magnetism. A controlled air gap exists between a friction surface on the armature, and a corresponding friction surface on a pole face for electromagnetic clutch windings located in the electromagnetic clutch housing. The electromagnetic clutch housing in turn is fixed to the transmission housing.
The electromagnetic clutch of the co-pending application lacks a clutch release feature that will maintain a desired air gap between the armature and the clutch face on the coil housing. Because of frictional drag that may be present following a command for disengagement of the electromagnetic brake, a clean release of the armature from frictional engagement with the coil housing following a braking cycle may not occur. Further, the spacing between the armature and the friction surface on the coil housing may not always be within design limits, following repeated application and release of the friction brake, since the flexible drive plate to which the armature is connected may not be capable of maintaining a controlled air gap when the electromagnetic brake is de-energized. Further, the release feature for the electromagnetic brake may not accommodate increases in the air gap due to friction surface for wear over the life of the brake.